Challenges with Electromechanical Scooter Starter Solenoids
Electromechanical scooter starter solenoids, although inexpensive, suffer from reliability issues over time. Problems such as increasing contact resistance, open circuits in the coil due to vibrations, and occasional welding of power contacts can occur, diminishing their effectiveness.
A Solution: Solid State Relays
To address the shortcomings of electromechanical solenoids. A viable solution involves replacing them with solid-state relays. These components provide a more dependable alternative. In the context of DC applications, utilizing a MOSFET transistor is essential to achieve the desired functionality and reliability.
Selecting a P-Channel MOSFET for Automotive Systems
In automotive systems, it’s common for the supply negative to be connected to the chassis ground. In such scenarios, the appropriate choice for switching components is a P-channel MOSFET. The current that needs to be switched can be relatively high, ranging from 55 to 100 A, depending on factors such as engine capacity and compression. Therefore, it’s crucial to select a MOSFET with an extremely low RDS(on) capable of handling a large IDS (drain-source current). Additionally, as the starter is essentially a DC motor with brushes, it generates substantial voltage spikes that can be destructive to the driving components. As a result, comprehensive protection measures are necessary.
Introducing the Optoisolator Solution
A closer examination of the wiring diagrams for various scooters reveals that the safety switch on the brake supplies +12 V, while the starter button connects to ground. To resolve this, one effective solution is to incorporate an optoisolator. It’s noteworthy that this optoisolator-based circuit can be versatile and used for a variety of other applications. Moreover, it’s essential that the circuit be “Plug-n-Play,” meaning it should be compatible with the original connector, limiting its dimensions to 50 × 50 mm.
Calculations for Handling High Current
To create a PCB capable of handling a current of 70 A, specific calculations are needed. For instance, the resistance of a copper track with particular thickness, length, and width must be calculated. By considering component positions and employing a double-sided board, it’s possible to achieve low resistance and minimize voltage drop and power dissipation. A suitable MOSFET for the application, such as the SUP75P03-07-E3, can further enhance performance.
Circuit Diagram and Testing
The circuit diagram consists of several key elements. The 4N28 optoisolator, along with resistors R1 and R2, plays a crucial role in controlling the MOSFETs, ensuring they are activated when both starter motor contacts are closed. Protection components, including capacitors and diodes, are incorporated to safeguard the circuit from interference generated by non-resistive loads. The circuit’s performance was tested on a scooter equipped with a GY6 engine, resulting in successful and consistent operation with negligible heating even after extended testing periods.